1. Field of the Invention
The present invention relates to an improved circuit interrupting device, and, more particularly, to an improved, high voltage, fuse-like circuit interrupting device which utilizes relatively movable contacts and pressurized dielectric fluid to extinguish an arc incident to circuit interruption.
2. Description of the Prior Art
Various alternating current circuit interrupting devices are well known, including fuses, circuit breakers, reclosers, circuit switchers, and the like. The present invention relates to fuses or fuse-like devices, as opposed to other types of interrupting devices.
In general, high voltage fuses utilizing a movable contact, often termed an "arcing rod", are well known. Typically, as the arcing rod moves, an arc is established between the arcing rod and a stationary contact. Such fuses normally contain a solid arc-extinguishing material (often referred to as an "ablative material") such as boric acid, cellulose "horn" fiber or the like. Interaction of the elongating arc with the solid arc-extinguishing material generates large quantities of high dielectric strength arc-extinguishing gas which deionizes, cools and causes turbulence in the region occupied by the arc. These effects of the gas, in combination with the elongation of the arc as the arcing rod moves away from the stationary contact, extinguish the arc at a subsequent current zero. Typically, arcing rod movement is effected by a stored energy operator, such as a spring.
While prior art fuses of the type generally described above have been made to work well at 69 kV and below, when adapted for use at higher voltages, say 115 kV or higher, they become quite expensive and may operate inconsistently from fuse to fuse. Moreover, many prior art fuses are quite complicated and expensive, and many of the chemicals used as solid arc-extinguishing materials have recently become prohibitively priced or of uncertain availability.
Various prior art patents disclose fuses or fuse-like devices which are species of the fuses described above in general terms. This species of fuse utilizes movable contacts, stationary contacts, and various arc-extinguishing materials. The following patents disclose fuses in which a pressurized fluid is used as an arc-extinguishing medium, usually in addition to an ablative arc-extinguishing material: Ackermann U.S. Pat. No. 3,265,838; Link U.S. Pat. No. 3,771,089; Rawlins U.S. Pat. No. 2,343,422; McCloud U.S. Pat. No. 3,032,630; Frink U.S. Pat. No. 3,268,690; and Triplett U.S. Pat. No. 2,319,277 (commonly assigned herewith). Although the fuses of these patents vary in their specific details of construction, they share many structural and functional features. Generally speaking, the fluid is retained in a reservoir by a massive stopper or plug. Connected in electrical series with the stopper is a fusible element, which is also connected to a stationary contact. A movable contact or arcing rod is prevented from moving away from the stationary contact by the presence of the fusible element. The arcing rod may be hollow and communicate with, or may constitute, the reservoir. Movement of the arcing rod is effected by a spring or by the jet action of the fluid escaping from the reservoir.
When an overcurrent melts the fusible element, the arcing rod is released for movement. Also, when the fusible element melts, an arc is established either between the stationary contact and the stopper, or in the vicinity of the stopper. The arc either burns a hole through the stopper or its heat melts the stopper, to release the fluid. Arc elongation, due to arcing rod movement, and fluid flow extinguish the arc.
The above-described devices have at least three inexpedient characteristics. First, the fusible elements and the stoppers are in electrical series and both must carry the normal, continuous current flowing through the devices. As a consequence, different stoppers (i.e., different as to size, thickness and material), as well as different fusible elements, must be used at different current ratings of the devices. Accordingly, both the fusible element and the stopper determine the time-current characteristics of the fuses. Second, the stopper is burned through and melted only after the arc is established. The establishment of the arc is preceded by the melting of the fusible element. Thus, the resultant arc, and not the overcurrent per se, burns through and melts the stopper. As a consequence, the devices are somewhat slow to operate due to the time it takes for the sequential occurrence of the melting of the fusible element, the establishment of the arc, and the burning through by the arc of the stopper. Third, the stopper is rather massive (to resist both the melting effect of normal current and the force exerted by the pressurized fluid) and requires substantial energy to be melted.
Even more importantly, the repeatability and consistency (from sample-to-sample) and the predictability of operation of all of the above prior art devices are doubtful. In addition to the previously-noted deficiencies, when a fusible element is in electrical series with a stopper through which an arc must burn a hole, the repeatable formation from sample-to-sample of a sufficiently large hole cannot be assured. Specifically, if a number of similar prior art devices were to be subjected to similar normal and fault conditions, arc-burned holes having various sizes, shapes and locations would be produced, due primarily to the evanescent nature of arcs and the inability to predict just where they may "root". Ackermann notes that "the opening [i.e., the hole] . . . may under certain conditions be too small to overcome by [`jet action`] the forces which hold the capsule [i.e., cartridge] . . . in place." Additionally, such variations in hole formation may be exacerbated by the variable heating effects on the stoppers caused by the variable normal currents they continuously carry. Moreover, the use of stoppers, through which the arc is expected to burn a hole, to carry normal current in series with a fusible element limits the choice of ampere ratings and time-current curves severely, leading, as noted by Ackermann, to the use of different arc-burned-through stoppers at different continuous and interrupting current ratings of the devices.
Commonly-assigned, co-pending, commonly-filed U.S. patent applications Ser. No. 904144 , filed May 24, 1978 discloses a device which represents an improvement over the type of device described above. There, a thin fusible diaphragm reliably and predictably melts to permit pressurized dielectric fluid flow from a normally closed reservoir and to release an arcing rod normally restrained by the diaphragm to the action of a spring. Most of the current flowing through the device is normally shunted away from the diaphragm by a fusible element, thus permitting the diaphragm to assume sufficient thinness to assure its reliable melting. When the fusible element melts, all of the current flowing through the device is instantaneously directed through the diaphragm to melt it. The fluid flow and elongation of an arc struck between the moving arcing rod and a stationary contact extinguish the arc.
Five commonly-assigned U.S. Pat. Nos. 2,571,735 (to Lindell); 2,517,624 (to Baker); 2,353,528; 2,319,277 (FIG. 12); and 2,319,276 (all to Triplett) relate to fuses in which a compressed dielectric fluid flows from a cartridge to aid in arc-extinguishment. Specifically, these patents disclose power fuses in which a spring-driven arcing rod moves away from a stationary contact following the melting of a fusible-element-strain-wire combination due to an overcurrent through the fuse. The movement of the arcing rod is effective to cause a pin or lance to move against the outside of, and to puncture, a seal closing the cartridge. An arc struck between the moving arcing rod and the stationary contact is extinguished by the combined action of the fluid from the cartridge, a gas evolved from a solid arc-extinguishing material with which the arc interacts, and arc elongation.
At first glance, the devices of these five patents might seem to eliminate one of the major disadvantages of the prior art devices utilizing a fusible stopper. Specifically, puncturing of a seal may appear to offer more reliable and predictable formation of a sufficiently large hole in the seal to ensure an instantaneous flow of fluid in large quantity. However, the entry of the pin into the seal from the outside thereof in part blocks the hole so formed, interfering with fluid flow. Hollow pins used to alleviate this shortcoming also block the hole, there being limitations on the size of passageways in the pins due to the limited amount of space available in the fuse. Also, experience has shown that the pins--solid or hollow--often fail to puncture the seal due to the limited amount of driving force available from realistic springs.
In most of the prior art devices where pressurized dielectric gas or fluid has been used, the gas or fluid is specifically effective only for the interruption of low fault currents. This is emphasized by the presence in the Ackermann, Baker, Frink, Lindell, Link, Rawlins and Triplett (all three) patents of both contained pressurized fluid and a solid or ablative arc-extinguishing material, interaction of which with the arc produces (in addition to the fluid) arc-extinguishing gas which is primarily effective at high fault current levels.
Accordingly, an overall object of the present invention is to obviate the above-noted problems inherent in the above-noted prior art devices.
A further object of the present invention is the provision of a fuse-like current interrupting device which
(a) is simple, inexpensive to manufacture and predictably reliable in operation;
(b) works reliably above 69 kV;
(c) does not require an ablative arc-extinguishing material;
(d) does not rely on the burning or melting of a hole to release an arc-extinguishing fluid; and
(e) avoids blockage of a hole formed in a seal by puncturing thereof.